1. Field of the Invention
The present invention relates generally to processes for bonding elastomers to metals and, more specifically, to a process which employs curable adhesives.
2. Description of the Related Art
There are numerous mechanical structures, particularly in the areas of shock and vibration dampening, wherein it becomes necessary to either adhere an elastomer to a metal substrate, or to interconnect two metal structures with an elastomer joint in which the elastomer is bonded to the surfaces of the two metal structures.
One type of structure is illustrated in FIG. 1, whereby a first metal part 10 is connected to a second metal part 12 by an elastomer joint 14. The metal part 10 has an arm 16 which extends between the arms 18 and 20 of a clevis-type structure.
In the past, to interconnect metal parts to an elastomer, the elastomer could be pre-vulcanized (meaning that an elastomeric block 14 could be fabricated prior to attachment) and then a low-temperature adhesive cure cycle would be used to attach the elastomeric block to a metal structure. For example, in the illustration of FIG. 1, the arm 18 and the elastomeric block 14 (in a pre-vulcanized condition) could be adhered together by applying an adhesive on the opposing surfaces of each. After curing, the interconnected structure would consist of a metal arm 18, an elastomeric block 14, and an intermediate thin layer of cured adhesive (not shown).
It has also been known in the past to vulcanize the elastomer 14 to a preheated adhesive surface. In other words, the various metal parts 10 and 12 could be placed in a mold cavity after applying an adhesive to the surfaces of the metal parts which would be in contact with elastomeric material, and then vulcanization would occur whereby elastomeric material is injected in to the mold at high pressure and temperature.
Disadvantages of the prior art are evident in the not so infrequent separation (de-bonding) of the metal parts from the corresponding elastomeric material. This can be a particularly hazardous problem when the metal parts are used in the aircraft industry, such as in the area of a helicopter rotor. In the one instance, prevulcanized elastomeric material adhered by an adhesive to the metal parts has been found to cause premature de-bonding and thus it can be said that a weak bond is created at the interface between the metal and the elastomer. Also, pre-vulcanizing results in excessive time and labor due to the numerous process steps individually required to accomplish the bond. Moreover, when separate vulcanization and bonding cycles are used, additional thermal and physical stresses on the materials and interfaces are imposed, thus enhancing the likelihood of a part failure such as premature de-bond.
Other bonding techniques have been noted from various U.S. patents. A brief description follows:
U.S. Pat. No.2,234,621, issued to Brous, discloses a method of adhering polyvinyl chloride to metal by means of a layer of halogenated rubber between the two materials. The process uses a heated press, but does not describe a situation where an adhesive and an elastomer are co-cured.
U.S. Pat. No. 2,765,248, issued to Beech et al, discloses a combined metal and plastic article formed in a mold, with the plastic used as a backing for a thin metal shell. The plastic is formed in the metal shell while the metal shell is still hot so that the resin tends to fill the open pores of the metal to form a band between the plastic and the shell.
U.S. Pat. No. 3,207,358, issued to Fliss, discloses a water storage tank that includes an internal plastic liner. A steel tank body is cleaned, grit-blasted, and then pre-heated. A spray gun uniformly applies an epoxy resin to the tank body. The epoxy resin is fully cured and becomes strongly adherent on the inner surface of the steel tank body by heating both the tank and the resin.
U.S. Pat. No. 3,243,321, issued to Rowand discloses a method of coating metal surfaces with polytetrafluoroethylene (PTFE).
U.S. Pat. No. 3,481,812, issued to Holub et al, discloses a method of laminating metal substrates with a polymer of ethylene, particularly polyethylene. A peroxide is incorporated in the polyethylene so that it adheres to the metal surface during curing.
U.S. Pat. No. 3,514,312, issued to Gardiner, discloses a method of bonding plastisol material to a metal substrate. An adhesive coating on the metal substrate is heated before bonding.
U.S. Pat. No. 4,233,098, issued to Urbain, discloses a method of making metal-plastic skis which includes glueing coated metal sheets to a plastic material. The plastic and adhesive are not cured during the bonding process.
In none of the above patents are there any methods for molding an elastomer on a metal substrate with an adhesive, with simultaneous curing/vulcanization of the adhesive and elastomer. In summary, the previous methods of bonding elastomers to metal substrates include either using a low-temperature adhesive cure cycle while a previously vulcanized elastomer is bonded, or vulcanizing the elastomer occurs during the bonding process with a previously cured (pre-heated) adhesive surface. Both of these methods cause premature debonding of parts because of a weak bond interface between the metal and the elastomer. In the latter, the common practice is to place the mold containing the metal substrate in the press while the press plates are being heated so that the adhesive begins to cure prior to injection of the elastomeric material.